The neurotrophins play a vital role in neuronal differentiation and maintenance. Though the low and high affinity neurotrophin receptors (p75NGFR and Trk proteins, respectively) have been isolated, their specific functions remain unclear. They hypothesis that p75NGFR and TrkA interact to form the high affinity NGF receptor holds true in some cell types, while others, only TrkA is required, challenging the proposed role of p75NGFR. The intracellular environment progressively changes during neuronal differentiation; though presumptive, the functional interactions of p75NGFR and TrkA may change dependent upon alterations in the cellular milieu. That is, these proteins may interact as a receptor complex at one stage of differentiation while showing convergent, parallel or even divergent actions in response to NGF at another. In this proposal, the expression of these receptors will be specifically inhibited by stably transfecting PC12 cells with vectors encoding antisense p75NGFR or trkA cRNA whose expression is controlled by inducible promoters. This will allow the dissociation of the receptor molecules in an appropriate intracellular environment to adequately address their individual, temporal functions. Since each receptor mRNA is further induced in response to NGF, it will be essential to investigate the role of these receptors in later stages of neuronal differentiation. In addition to acting as a differentiative factor, neurotrophins are also necessary for the survival of specific neuronal populations. Though NGF causes rapid transcriptional induction generating nascent proteins necessary for differentiation, the hormone also has effects upon mRNA survival. This aspect of NGF's actions likely represents a function of neurotrophin-dependent phenotypic maintenance. PC12 cells continually express and accumulate mRNAs necessary for transcriptional-independent neuritic reextension in the event of injury. Those mRNAs whose stability is increased by NGF will be identified and characterized. Since transcriptional activation represents a distinct intracellular mechanism from mRNA degradation, it will be essential to elucidate the signal transduction cascade involved. While much current work addresses the molecular mechanisms of rapid mRNA degradation, that of enhanced mRNA survival has yet to be adequately studied. The neurotrophins also function in neuronal regeneration and degeneration. The understanding of their mechanisms of action will provide invaluable insight to the possible prevention and treatment of peripheral neuropathies and neurodegenerative disorders. Thus, it will be essential to extend the observations of mRNA stability to in vivo models of regeneration and aging.